Submersible centrifugal pumps consist of a pump, and an electric motor suitable for operating submerged in water for driving the pump. Present designs use conventional induction type electric motors, which usually operate at two-pole speeds, i.e., 3,500 rpm. At speeds higher than 5,000 rpm, the efficiency of the more commonly used induction type motors begins to deteriorate.
The performance of a centrifugal pump is directly related to the running speed. The higher the running speed, the more pressure is developed, and a higher flow rate is generated. The pressure will increase by the square of the speed ratio while the flow rate will increase in direct proportion to the change in speed. Thus, if the running speed was changed from 3,500 to 10,000 rpm, the pressure developed would increase by a factor of 8.16, and the flow rate by 2.86. To meet the same requirements, the higher speed pump can be manufactured much smaller in physical size. This reduction in size translates to savings in material, and allows installation of the pump into smaller spaces.
At 10,000 rpm, the efficiency of an induction type motor becomes unacceptable. The alternative is an electric motor employing permanent-magnets, either in the rotor or stator, or both. This type of electric motor has excellent efficiency at high speeds, and does not incur as much induction losses as, for example, a squirrel cage induction motor. The magnetic flux is always preset, and does not have to be created by induction. For simplicity, the rotor almost always has the permanent-magnets, thus negating the need to commutate the electric power to the rotor. Usual prior art rotors are constructed as a solid piece, and then magnetized.
Rare-earth magnets have been developed, such as neodymium-ferrite-boron, which provide further efficiencies and size reduction. Such materials, however, are relatively expensive when incorporated into electric motors.
Prior art rotors are normally assembled as one solid piece, which is then magnetized. With rare-earth magnetic material, this construction is not economically feasible, or required.
The output power of a permanent-magnet electric motor is determined by the length of the stator and the corresponding rotor. For the same diameter of either rotor or stator, a larger axial length develops increased power, up to a reasonable limit. Therefore, if the rotor core consists of sections, or modules, the rotor length, and hence the output power, can be varied at the point of manufacture of the pump. Further, constructing the rotor core in modules allows the rotor to be produced using the powdered metal (sintered) process due to the relatively short length of each module. This produces significant cost reduction, as powdered metal technology lends itself to low cost, high volume production.
It is an object of the present invention to provide a modular permanent-magnet electric motor assembly, desirable for a submersible pump, wherein the power output of the motor can be variably selected at the manufacturing stage.
It is a further object of the present invention to provide a rotor for a permanent-magnet electric motor wherein the rotor is comprised of a plurality of modules, and the number of modules is determined by the desired power output of the motor. The length of the stator stack corresponds to the length of the rotor assembly.
A further object of the present invention is to provide a rotor utilizing thin sections of rare-earth magnets bonded to a rotor core to produce an efficient, less expensive and smaller size permanent-magnet electric motor.
Another object of the present invention is to provide a rare-earth magnet rotor assembly with reduced requirements of rare-earth materials while simultaneously allowing the flexibility to construct motors of varying power outputs.